Our backyard has about a 25% slope, and we didn't love the idea of putting the trampoline in the front yard, so I had to find a way to keep the trampoline level. Conduit turned out to be the ideal construction material. I made a set of support legs, and honestly they're probably sturdier than the trampoline's original legs.
The trampoline legs are bolted to pressure-treated lumber, and the lumber is attached to the conduit via flange fittings. 12" rebar U-stakes anchor the conduit to the ground. Probably the hardest part was just making all of the legs level; each one was at a slightly different height.